A passenger using an electronic cigarette which gives off a vapour caused armed police to swoop on a coach in a terror alert which closed a motorway for more than four hours.

Armed officers, troops, firefighters and bomb disposal experts all went to the scene on the M6 toll road near Lichfield after a passenger reported seeing vapour coming from a man's bag.

A decontamination unit was set up and passengers were searched and held in a makeshift pen on the carriageway, but police found no crime had been committed. The man was simply using an electronic cigarette, rather than the real thing, on the six-hour journey from Preston to London.

With the Olympic Games, considered an attractive target for potential terrorists, starting later this month, police have urged the public to remain vigilant and report any suspicious activity.

Officers responded "swiftly and proportionately" to "a genuine report from a member of the public", a Staffordshire Police spokeswoman said. "The information received concerned a report of vapour escaping from a bag which on investigation turned out to be a health improvement aid for smokers."

Police later confirmed the device was an electronic cigarette which gives off a visible vapour.

"We can now confirm that, whilst this was a genuine security alert, the significant concerns reported to us were unfounded," the force spokeswoman said. "It's important to state that no criminal offence has been committed and no passenger or any other member of the public is being treated as a suspect."

Police received a report at about 8.20am on Thursday from "a genuinely concerned member of the public" who saw vapour coming out of the bag as the coach was near the M6 toll plaza at Weeford, near Lichfield. The incident shut the M6 toll road in both directions for more than four hours.

A spokeswoman for Megabus, which is operated by Stagecoach, said 48 passengers were booked on the service and all were safe and well. She said: "We are assisting police with their inquiries into an allegation made against a passenger who was travelling on the 05.10 Preston to London service.

"There were 48 people booked to travel on board the service, which was due to arrive at Victoria coach station in London at 10.55am. Police have confirmed that all passengers are safe and well and they have been transferred to a substitute vehicle. We are making arrangements to provide onward travel for passengers as soon as we receive clearance from the police."